This invention relates to a rotating machine, and more particularly to a rotating machine whose start is effected by actuating only some of the coils which are all energized during the full drive.
Various methods have been proposed for the start of a rotating machine such as an electric motor. These methods include the type which effects the start of a rotating machine by actuating only some of the coils attached to the machine. This method comprises dividing all the coils which are to be energized during the full drive into two or circuits groups and energizing one or the other of these two or circuit groups to effect the start of the rotating machine.
FIGS. 1 and schematically indicate the conventional method which starts a 3-phase 36-slot 2-pole electric motor by actuating only some of the coils used. In FIGS. 1 and 2, the characters 1 to 36 denote the serial slot numbers of a stator (not shown). Coils are received in the slots in six groups in the following manner:
Slots 1 to 6 hold the first group of U-phase coils marked with ; PA1 Slots 7 to 12 hold the second group of Z-phase coils marked with ; PA1 Slots 13 to 18 hold the third group of V-phase coils marked with ; PA1 Slots 19 to 24 hold the fourth group of X-phase coils marked with ; PA1 Slots 25 to 30 hold the fifth group of W-phase coils marked with ; PA1 Slots 31 to 36 hold the sixth group of Y-phase coils marked with ;
Black marks given in FIGS. 1 and 2 denote the coils which are not energized at the start of a rotating machine. For briefness of illustration, FIGS. 1 and 2 indicate only the upper coils. The lower coils (not shown) are received in the corresponding slots in a state respectively displaced from the upper coils by a prescribed coil pitch.
The field winding of the electric motor of FIG. 1 is formed of U-, V- and W-phase coils so connected as to produce three magnetic fields spaced from each other at an electric angle of 60.degree. (FIG. 6) as counted in the circumferential direction of a cylindrical stator. The field winding of the electric motor of FIG. 2 is formed of U-, Z- and V-phase coils so connected as to produce three magnetic fields arranged adjacent to each other on one segmental section of the cylindrical stator jointly to define an electric angle of 180.degree. (FIG. 7).
FIGS. 3 and 4 respectively typically illustrate the manner in which the coils of the various phases shown in FIGS. 1 and 2 are connected. In FIGS. 3 and 4, the characters R.sub.1, R.sub.2, S.sub.1, S.sub.2, T.sub.1, T.sub.2 denote terminals for connecting the coils of the various phases. 3-phase A.C. current is supplied to the coils through said contacts.
FIG. 5 is a development diagram of coils held in the slots of the stator of the electric motor of FIGS. 1 and 3. A development diagram of the coils of the respective phases held in the slots of the stator of the electric motor of FIGS. 2 and 4 is omitted, because said coils are connected only in a different manner from FIG. 5.
Where 3-phase A.C. current is supplied to the coils of the U-, V- and W-phases to start the motor of FIGS. 1, 3 and 5, then the A.C. current flows through the coils U, V and W as indicated by the hatchings of FIG. 6. The remaining coils of the Z-, X- and Y-phases which are not supplied with the 3-phase A.C. current do not produce actual magnetic fields, but only imaginary ones. Each of these imaginary magnetic fields are formed by an opposite electromotive force induced by the magnetic fields generated by the coils of the U-, V- and W-phases, and have the opposite poles to those of the coils of the U-, V- and W-phases. At the start of the electric motor of the above-mentioned arrangement, the magnetic fields of U, V and W and imaginary magnetic fields Z, X and Y are produced. During the acceleration period after the start, therefore, an even harmonic magnetomotive force exerts a prominent effect on a fundamental harmonic magnetomotive force, eventually resulting in a decline in a magnetomotive force. Consequently a torque valley appears which results from a valley-like descent of a torque characteristic curve. Where said torque valley is positioned below a load torque, then the electric motor can not be accelerated.
A prior art electric motor in which the coils of the various phases are connected as shown in FIGS. 2 and 4 are accompanied with the drawbacks of a prior art electric motor described with reference to FIGS. 1 and 3. The hatching of FIG. 7 shows the coils which carry the A.C. current when 3-phase A.C. current is applied to the coils of the U-, Z- and V-phases of the electric motor of FIGS. 2 and 4. In this case, too, the actual magnetic fields U, Z, V and imaginary magnetic fields X, W, Y are produced, causing the even harmonic magnetomotive force to exert a noticeable effect on the fundamental harmonic magnetomotive force. Where, therefore, the resultant torque valley falls before the load torque, then the electric motor can not be accelerated as in the aforementioned case.